1. Field of the invention
The present invention relates to a method of analyzing hydrocarbon oil mixtures by gel-permeation chromatography.
More precisely, the present invention relates to a method whereby certain characteristics of a mixture of oils, such as the concentrations of the various different oils and/or the nature of one of the oils in the mixture can be determined.
The invention is particularly suitable for use in the oil industry, in particular in drilling and production fields.
2. Description of Prior Art
Petroleum oils are hydrocarbon oils constituted by mixtures of aliphatic hydrocarbons and aromatic hydrocarbons, and it is advantageous to be able to characterize and monitor such mixtures on site at exploration or production wells, and at sites where oil is transferred.
This type of monitoring can be performed by a spectrometer method using ultraviolet radiation as described in U.S. Pat. No. 4,609,821.
Gel-permeation is a technique that is widely used in the laboratory for analyzing petroleum oils or fractions. In this technique, the hydrocarbon oil is separated into fractions of different molecular masses, and these fractions may be identified at the outlet from the chromatography column by means of an appropriate detector, generally a diffractive index detector, an ultraviolet detector, or a flame ionization detector, as described in "Chromatography in petroleum analysis" Chromatographic Science Series, Vol. 11, pp. 287-294.
However, this technique is not easy to use for determining the "fingerprint" that enables a crude oil to be characterized, i.e. for establishing a curve showing the distribution of the constituents of the oil as a function of their molecular masses. Given that the detectors used have responses that vary depending on the substance analyzed, it is difficult to convert elution curves from a chromatography column into molecular mass distribution curves. As a result, this technique cannot be used for obtaining the molecular mass distribution of an oil sample without resorting to additional measurements as described at page 305 of the above-specified document.
The fingerprint of a crude oil is thus generally determined by gas chromatography making use of flame ionization detectors.
In "Journal of Chromatography", 312, 1984, pp. 261-272, P. Guieze and J. M. Williams describe the use of the gel-permeation technique for determining molecular mass in oil distillation residues by making use of three chromatography columns in series together with two detectors, namely an infrared detector and an ultraviolet detector for analyzing the fractions leaving the chromatography columns. In that case, a computer serves to convert the information coming from the two detectors into molecular masses.
That publication also mentions that the technique could be used for characterizing petroleum oils by means of the fingerprints provided by the two detectors, but until now, nobody has envisaged using gel-permeation for obtaining such fingerprints.